Optical microresonator

ABSTRACT

An optical device and a sensor system incorporating same are disclosed. The optical device includes a microresonator that has a core with input and output ports. The output port is different than the input port. The optical device further includes first and second optical waveguides. Each optical waveguide has a core with input and output faces. The output face of the core of the first optical waveguide physically contacts the input port of the core of the microresonator. The input face of the core of the second optical waveguide physically contacts the output port of the core of the microresonator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/565,935, filed on Dec. 1, 2006, now allowed, the disclosure of whichis incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

This invention generally relates to optical devices. The invention isparticularly applicable to optical devices such as optical sensors thatincorporate microresonators.

BACKGROUND

Microresonators have received increasing attention in variousapplications such as optical switching described in, for example, U.S.Pat. No. 6,876,796; optical filtering described in, for example, U.S.Pat. No. 7,092,591; wavelength filtering described in, for example, U.S.Pat. No. 7,062,131; optical lasers described in, for example, U.S. Pat.No. 6,741,628; light depolarization described in, for example, U.S. Pat.No. 6,891,998; and chemical and biological sensing described in, forexample, U.S. Pat. No. 5,744,902.

Some known microresonator constructions involve placing a glassspherical microresonator in close proximity to an optical waveguide suchas an optical fiber. In such cases, optical energy can transfer betweenthe resonator and the optical waveguide by evanescent coupling. Theseparation between the resonator and the optical waveguide is typicallyless than one micron and must be controlled with precision to providereproducible performance. Other forms of microresonators include disk-or ring-shaped microresonators described in, for example, U.S. Pat. No.7,095,010.

SUMMARY OF THE INVENTION

Generally, the present invention relates to optical devices. The presentinvention also relates to optical sensors that include one or moremicroresonators.

In one embodiment, an optical device includes a microresonator that hasa core with input and output ports. The output port is different thanthe input port. The optical device further includes first and secondoptical waveguides. Each optical waveguide has a core with input andoutput faces. The output face of the core of the first optical waveguidephysically contacts the input port of the core of the microresonator.The input face of the core of the second optical waveguide physicallycontacts the output port of the core of the microresonator.

In another embodiment, an optical device includes a microresonator thathas a circular symmetry. The microresonator has a core. The opticaldevice further includes an optical waveguide having a core. Thewaveguide core terminates at the core of the microresonator.

In another embodiment, an optical device includes a light source, anoptical detector, and a microresonator that is capable of supportingfirst and second guided counter traveling optical modes. The secondguided optical mode is different than the first guided optical mode. Themicroresonator has a core with input and output ports where the outputport is different than the input port. The microresonator is capable ofbonding with an analyte associated with a scattering center. The opticaldevice further includes a first optical waveguide that has a core withan input face in optical communication with the light source and anoutput face in physical contact with the input port of the core of themicroresonator. The optical device further includes a second opticalwaveguide that has a core with an input face physically contacting theoutput port of the core of the microresonator and an output face inoptical communication with the optical detector. When the associatedanalyte bonds with the microresonator, the scattering center is capableof inducing an optical scattering between the first and second guidedoptical modes. The optical scattering results in a transfer of energyfrom the first guided mode to the second guided mode. The opticaldetector detects the transfer of energy.

In another embodiment, an optical device includes a microresonator thatis capable of supporting at least two resonant optical modes. At leastone of the two resonant modes is capable of propagating within themicroresonator while maintaining a same electric field profile. Theoptical device further includes first and second optical waveguides thatare capable of coupling to the microresonator by core coupling.

In another embodiment, an optical device includes a microresonator thathas a core. The optical device further includes a first opticalwaveguide that has a core that extends from a first location on the coreof the microresonator. The optical device further includes a secondoptical waveguide that has a core that extends from a second location onthe core of the microresonator. The second location is different fromthe first location.

BRIEF DESCRIPTION OF DRAWINGS

The invention may be more completely understood and appreciated inconsideration of the following detailed description of variousembodiments of the invention in connection with the accompanyingdrawings, in which:

FIGS. 1 and 2 are respective schematic top- and side-views of an opticaldevice;

FIGS. 3-5 are schematic top-views of optical devices with differentarrangements of optical waveguides;

FIGS. 6 and 7 are schematic top-views of optical devices with variousclosed loop microresonators;

FIG. 8 is a schematic top-view of an optical device;

FIG. 9 is a schematic top-view of an optical device with a singleoptical waveguide;

FIG. 10A is a plot of a calculated output signal strength as a functionof wavelength;

FIG. 10B is a magnified view of a portion of the plot in FIG. 10A;

FIG. 11 is a schematic three-dimensional view of an integrated opticaldevice;

FIGS. 12A and 12B are respective schematic top- and side-views of anoptical device with vertical core coupling;

FIG. 12C is a schematic top-view of an optical device with vertical corecoupling;

FIGS. 13A and 13B are respective schematic top- and side-views of anoptical device with vertical core coupling; and

FIG. 14 is a schematic three-dimensional view of an optical device.

In the specification, a same reference numeral used in multiple figuresrefers to the same or similar elements having the same or similarproperties and functionalities.

DETAILED DESCRIPTION

This invention generally relates to optical devices. The invention isparticularly applicable to optical devices such as optical sensors thatincorporate microresonators.

The present invention describes an optical device that includes one ormore waveguides optically coupled to an optical microresonator. Theperformance of the disclosed embodiments is relatively insensitive tothe placement of the optical waveguide(s) relative to the opticalmicroresonator. As such, the present invention can reduce manufacturingcosts since, for example, manufacturing errors and/or limitations inplacing the optical waveguide(s) in optical proximity with the opticalmicroresonator are less likely to result in a substantial change in theoptical coupling.

FIGS. 1 and 2 show schematic top- and side-views of an optical device100, respectively. Optical device 100 includes an optical microresonator110, a first optical waveguide 120, and a second optical waveguide 130all disposed on a lower cladding layer 165 disposed on a substrate 161.

In some cases, microresonator 110 is capable of quantizing the allowedoptical modes of the microresonator into discrete modes by imposing oneor more boundary conditions, such as one or more periodicity conditions.In some cases, microresonator 110 is capable of supporting at least twodifferent guided optical modes such as first guided optical mode 150 andsecond guided optical mode 152, where guided optical mode 152 isdifferent than guided optical mode 150. In some cases, modes 150 and 152have the same wavelength.

As used herein, for a given optical configuration such as optical device100, an optical mode refers to an allowed electromagnetic field in theoptical configuration; radiation or radiation mode refers to an opticalmode that is unconfined in the optical configuration; a guided moderefers to an optical mode that is confined in the optical configurationin at least one dimension due to the presence of a high refractive indexregion; and a resonant mode refers to a guided mode that is subject toan additional boundary condition requirement in the opticalconfiguration, where the additional requirement is typically periodic innature.

Resonant modes are typically discrete guided modes. In some cases, aresonant mode can be capable of coupling to a radiation mode. In someother cases, a resonant mode can have a component that is radiation andis not confined. In general, a guided mode of microresonator 110 can bea resonant or a non-resonant mode. For example, optical modes 150 and152 can be resonant modes of microresonator 110.

In some cases, first guided optical mode 150 and/or second guidedoptical mode 152 is capable of propagating within the microresonatorwhile maintaining a same electric field profile. In such cases, theshape or profile of the propagating mode remains substantially the sameeven if the mode gradually loses energy because of, for example,absorption or radiation losses.

In general, microresonator 110 may be single mode or multimode along aparticular direction. For example, microresonator 110 can be single ormultimode along the thickness direction (e.g., the z-direction) of themicroresonator. In some cases, such as in the case of a sphere- ordisc-shaped microresonator, the microresonator can be single ormultimode along a radial direction. In some cases, such as in the caseof a disk-shaped microresonator, guided optical modes 150 and 152 ofmicroresonator 110 can be azimuthal modes of the microresonator.

Microresonator 110 includes a core or cavity 112 disposed between lowercladding 165 and an upper cladding 114. Core 112 has an averagethickness h₁. In general, for an electric field associated with a modeof microresonator 110, the evanescent tails of the field are located inthe cladding regions of the microresonator and the peak(s) or maxima ofthe electric field are located in the core region of the microresonator.For example, as schematically shown in FIG. 2, a guided mode 151 ofmicroresonator 110 has an evanescent tail 151A in upper cladding 114, anevanescent tail 151B in lower cladding 165, and a peak 151C in core 112.Guided optical mode 151 can, for example, be mode 150 or 152 of themicroresonator.

In the exemplary optical device 100, core 112 is disposed between twocladding layers 114 and 165. In general, microresonator 110 can have oneor more upper cladding layers and one or more lower cladding layers. Insome cases, lower cladding layer 165 may not be present in opticaldevice 100. In such cases, substrate 161 can be a lower cladding layerfor microresonator 110. In some other cases, microresonator 110 does notinclude upper cladding layer 114. In such cases, an ambient medium, suchas ambient air, can form the upper cladding of the microresonator.

Core 112 has an index of refraction n_(m), cladding 114 has an index ofrefraction n_(uc), and cladding 165 has an index of refraction n_(lc).In general, n_(m) is greater than n_(uc) and n_(lc) for at least onewavelength of interest and along at least one direction. In someapplications, n_(m) is greater than n_(uc) and n_(lc) in a wavelengthrange of interest. For example, n_(m) can be greater than n_(uc) andn_(lc) for wavelengths in a range from about 400 nm to about 1200 nm. Asanother example, n_(m) can be greater than n_(uc) and n_(lc) forwavelengths in a range from about 700 nm to about 1500 nm.

Microresonator core 112 has an input port 115A and an output port 115B,where output port 115B is different than input port 115A. For example,in the exemplary optical device 100, input port 115A and output port115B are located at different locations around an outer surface 116 ofcore 112.

Each of the first and second optical waveguides 120 and 130 has a coredisposed between multiple claddings. For example, first opticalwaveguide 120 has a core 122 having a thickness h₂ and disposed betweenupper cladding 114 and lower cladding 165. Similarly, second opticalwaveguide 130 has a core 132 having a thickness h₃ disposed betweenupper cladding 114 and lower cladding 165.

Core 122 has an index of refraction n_(w1) which is, in general, greaterthan n_(uc) and n_(lc). Similarly, core 132 has an index of refractionn_(w2) which is, in general, greater than n_(uc) and n_(lc).

In some cases, cores 112, 122, and 132 may be made of different corematerials having the same or different indices of refractions. In someother cases, cores 112, 122, and 132 may form a unitary construction,meaning that the cores form a single unit with no physical interfacesbetween connecting cores. In a unitary construction, the cores may bemade of the same core material. A unitary construction can be made usinga variety of known methods such as etching, casting, molding, embossing,and extrusion.

Core 122 has an input face 122A and an output face 122B. Input face 122Ais in optical communication with a light source 140. Output face 122Bphysically contacts input port 115A of core 112. In some cases, such asin a unitary construction, output face 122B can be the same as inputport 115A. In some cases, there is significant overlap between outputface 122B and input port 115A. In some cases, one of output face 122Band input port 115A completely covers the other. For example, in somecases, output face 122B is larger than and completely covers input port115A of the microresonator.

Core 132 has an input face 132A and an output face 132B. Output face132B is in optical communication with an optical detector 160. Inputface 132A is in physical contact with output port 115B of core 112 ofmicroresonator 110.

Light source 140 is capable of emitting light beam 142, at least aportion of which enters first optical waveguide 120 through input face122A. In some cases, light entering optical waveguide 120 from lightsource 140 can propagate along the waveguide as a guided mode of thewaveguide. First optical waveguide 120 and input port 115A are sopositioned, for example, relative to each other and/or themicroresonator, that light traveling in first optical waveguide 120along the positive y-direction toward input port 115A is capable ofcoupling primarily to first guided optical mode 150 of themicroresonator but not to second guided optical mode 152 of themicroresonator. For example, light propagating along optical waveguide120 and reaching output face 122B is capable of exciting primarily firstguided optical mode 150 but not second guided optical mode 152. In somecases, there may be some optical coupling between light propagating inoptical waveguide 120 and guided optical mode 152. Such coupling may beby design or due to, for example, optical scattering at input port 115A.As another example, such coupling may be due to optical scattering frommanufacturing or fabrication defects. In cases where there is someoptical coupling between light propagating in optical waveguide 120 andguided optical mode 152, the propagating light primarily couples tooptical mode 150.

Second optical waveguide 130 and output port 115B are so positioned, forexample, relative to one another and the microresonator, that lighttraveling in second optical waveguide 130 along the positive y-directionaway from output port 115B is capable of coupling primarily to secondguided optical mode 152 of the microresonator but not to first guidedoptical mode 150 of the microresonator. For example, guided mode 152 ator near output port 115B is capable of exciting a guided mode 133 in thesecond optical waveguide propagating along the positive y-directiontoward output face 132B. In contrast, guided optical mode 150 is notcapable of or is weakly capable of exciting guided mode 133. In somecases, there may be some optical coupling between guided optical mode150 and guided mode 133 due to, for example, optical scattering atoutput port 115B. But any such coupling is secondary to the opticalcoupling between guided modes 152 and 133.

In the exemplary optical device 100 of FIGS. 1 and 2, microresonator 110and optical waveguides 120 and 130 have different thicknesses. Ingeneral, thicknesses h₁, h₂, and h₃ may or may not have the same value.In some applications, microresonator 110 and optical waveguides 120 and130 have the same thickness.

Optical waveguides 120 and 130 can be any type of waveguide capable ofsupporting an optical mode, such as a guided mode. Optical waveguides120 and 130 can be one-dimensional waveguides such as planar waveguides,where a one-dimensional waveguide refers to light confinement along onedirection. In some applications, optical waveguides 120 and 130 can betwo-dimensional waveguides where a two-dimensional waveguide refers tolight confinement along two directions. Exemplary optical waveguidesinclude a channel waveguide, a strip loaded waveguide, a rib or ridgewaveguide, and an ion-exchanged waveguide.

In the exemplary optical device 100, core 122 of first optical waveguide120 and core 132 of second optical waveguide 130 are substantiallyparallel at or near their respective contact points with themicroresonator. In particular, both cores 122 and 132 extend along they-axis at contact points 115A and 115B, respectively. Cores 122 and 132,however, are not collinear. In particular, core 132 is offset relativeto core 122 along the x-axis. In general, cores 122 and 132 may or maynot be parallel at the input and output ports. Similarly, cores 122 and132 may or may not be collinear at the input and output ports. Forexample, in FIG. 3A core 132 is oriented along the x-axis at output port115B and core 122 extends along the y-axis at input port 115A eventhough core 122 eventually bends toward the x-axis. As another example,in FIG. 3B core 122 is along the y-axis at input port 115A and core 132makes an angle a with the y-axis at output port 115B, where the absolutevalue or magnitude of a can be in a range from about zero degrees toabout 180 degrees. In general, α can be positive or negative. Therefore,in general, α can be from about −180 degrees to about 180 degrees. Forexample, α can be about 45 degrees.

In the exemplary optical devices of FIGS. 1-3, the cores of the twooptical waveguides are tangentially connected to the core of the opticalmicroresonator. In general, a core of an optical waveguide may bephysically connected to a core of an optical microresonator in any waythat may be suitable in an application. For example, FIG. 4 shows anoptical device 400 having cores 122 and 132 of optical waveguides 120and 130 attached to core 112 of microresonator 110 at attachmentlocations 401 and 402, respectively. Core 122 intersects core 112 atattachment location 401 and makes an angle β₁ with line 410 tangent tocore 112 at location 401. Similarly, core 132 intersects core 112 atattachment location 402 and makes an angle β₂ with line 420 tangent tocore 112 at location 402. Angles β₁ and β₂ may or may not be equal.Angles β₁ and β₂ may be any angle that may be desirable in anapplication. In some applications, angles β₁ and β₂ are in a range fromabout zero degrees to about 45 degrees. In some other applications,angles β₁ and β₂ are in a range from about zero degrees to about 20degrees. In some other applications, angles β₁ and β₂ are in a rangefrom about zero degrees to about 10 degrees. In still some otherapplications, angles β₁ and β₂ are in a range from about zero degrees toabout 5 degrees.

In some cases, at least one of first and second guided optical modes 150and 152 can be a traveling guided mode of microresonator 110. Forexample, first and second guided optical modes 150 and 152 may be“whispering gallery modes” (WGMs) of microresonator 110. A WGM isgenerally a traveling mode confined close to the surface of amicroresonator cavity and has relatively low radiation loss. Since theWGMs are confined near the outer surface of the core of amicroresonator, they are well-suited to optical coupling with analyteson or near the microresonator surface.

Traveling guided optical modes 150 and 152 can propagate in different,for example opposite, directions. For example, in a disk or spheremicroresonator, first guided optical mode 150 can generally travel in acounter clockwise direction and second guided optical mode 152 cangenerally travel in a clockwise direction. In such a case, first andsecond guided optical modes 150 and 152 are counter-propagating opticalmodes.

In some cases, at least one of first and second guided optical modes 150and 152 can be a standing-wave mode of microresonator 110. Astanding-wave mode can be formed by, for example, a superposition of twotraveling modes having a proper phase relationship. In some cases, oneof the two traveling modes can be a reflection of the other travelingmode.

Light propagating in optical waveguide 120 along the positivey-direction couples primarily to first guided optical mode 150 ofmicroresonator 110. Since core 122 is physically connected to core 112,the optical coupling between first optical waveguide 120 andmicroresonator 110 is primarily a core coupling and not an evanescentcoupling.

An advantage of the present invention is elimination of a coupling gapbetween at least one optical waveguide and a microresonator. In knownmicroresonators, a gap exists between an optical waveguide and amicroresonator. In such cases, the optical coupling between thewaveguide and the microresonator is achieved by evanescent coupling.Such a coupling is very sensitive to, among other things, the size ofthe coupling gap which is typically hard to reproducibly control becauseof, for example, fabrication errors. Even in fabrication methods wherethe gap can be controlled with sufficient accuracy, such a control cansignificantly increase the manufacturing cost. In the present invention,the coupling gap is eliminated by providing direct physical contactbetween the core of an optical waveguide and the core of an opticalmicroresonator. This can result in reduced manufacturing cost andimproved reproducibility.

In some cases, first guided optical mode 150 is launched withinmicroresonator 110 when light from light source 140 enters waveguide 120through input face 122A and propagates to input port 115A ofmicroresonator 110. When a scattering center 170 is brought sufficientlyclose to microresonator 110, the scattering center induces an opticalscattering between first and second guided optical modes 150 and 152,respectively, resulting in a transfer of energy, or a change in transferof energy, from guided mode 150 to guided mode 152. If guided mode 152is already excited in microresonator 110, then the scattering centerresults in a stronger and more intense guided optical mode 152. Ifguided mode 152 is not already present in microresonator 110, then thescattering center induces a launching of guided mode 152 by causingoptical scattering from first guided mode 150 into second guided mode152. Guided mode 152 optically couples to optical waveguide 130 atoutput port 115B resulting in light propagating in waveguide 130 towardoutput face 132B. Detector 160 detects the transfer of energy betweenguided modes 150 and 152 and by doing so, is capable of detecting thepresence of scattering center 170.

When scattering center 170 is removed from optical proximity tomicroresonator, the removal induces a change in the optical scatteringbetween first and second guided optical modes 150 and 152, respectively,resulting in a change in transfer of energy from guided mode 152 toguided mode 150. Detector 160 detects the change in transfer of energyfrom guided mode 152 to guided mode 150 and by doing so, is capable ofdetecting the removal of scattering center 170.

A change in the strength of optical coupling between scattering center170 and microresonator 110 can induce a change in the optical scatteringbetween first and second guided optical modes 150 and 152, respectively.The change in the strength of optical coupling can be achieved byvarious means. For example, a change in the spacing “d” betweenscattering center 170 and microresonator 110 or core 112 can change thestrength of optical coupling between the scattering center and themicroresonator. As another example, a change in the index of refractionn_(s) of the scattering center can change the strength of opticalcoupling between the scattering center and the microresonator. Ingeneral, any mechanism that can cause a change in the strength ofoptical coupling between scattering center 170 and microresonator 110can induce a change in the optical scattering between guided modes 150and 152.

Optical device 100 can be used as a sensor, capable of sensing, forexample, an analyte 172. For example, microresonator 110 may be capableof bonding with analyte 172. Such bonding capability may be achieved by,for example, a suitable treatment of the outer surface of microresonator110. In some cases, analyte 172 is associated with scattering center170. Such an association can, for example, be achieved by attaching theanalyte to the scattering center. The scattering center may be broughtin optical proximity to microresonator 110 when analyte 172 bonds withthe outer surface of the microresonator. The scattering center inducesan optical scattering between first guided optical mode 150 and secondguided optical mode 152. The optical scattering results in a change intransfer of energy between the two modes. Optical detector 160 candetect the presence of analyte 172 by detecting the change in transferof energy between guided modes 150 and 152. Analyte 172 can, forexample, include a protein, a virus, or a DNA.

In some cases, analyte 172 can include a first antibody of an antigenthat is to be detected. The first antibody can be associated withscattering center 170. A second antibody of the antigen can beassociated with microresonator 110. The antigen facilitates a bondingbetween the first and second antibodies. As a result, the scatteringcenter is brought into optical contact with the microresonator andinduces a change in optical scattering within the microresonator. Thedetector can detect the presence of the scattering center, andtherefore, the antigen, by detecting the change in optical scattering.In some cases, the first antibody can be the same as the secondantibody. Such an exemplary sensing process can be used in a variety ofapplications such as in food safety, food processing, medical testing,environmental testing, and industrial hygiene.

In some cases, scattering center 170 can induce a frequency shift insecond optical mode 152 where the shift can be detected by detector 160.In some cases, scattering center 170 can induce a frequency shift infirst guided optical mode 150. In such cases, detector 160 can besufficiently sensitive and/or output port 115B can be sufficientlycapable of scattering mode 150 into a mode of waveguide 130, so thatdetector 160 can be capable of detecting the frequency shift in guidedmode 150.

FIG. 5A shows a schematic top-view of an optical device 520. In opticaldevice 520, second optical waveguide 130 and output port 115B are sopositioned relative to, for example, one another and/or themicroresonator, that light traveling in second optical waveguide 130along the negative y-direction away from output port 115B is capable ofcoupling primarily to first guided optical mode 150 of themicroresonator but not to second guided optical mode 152 of themicroresonator. For example, guided mode 150 at or near output port 115Bis capable of exciting a guided mode 533 in the second optical waveguidepropagating along the negative y-direction toward output face 132B. Insome cases, any optical coupling between guided optical mode 152 andguided mode 533 is substantially weaker than the optical couplingbetween guide modes 150 and 533. In some cases, the optical couplingbetween guided optical mode 152 and optical waveguide 130 may besufficiently strong and/or detector 160 may be sufficiently sensitive soas to permit detection of the optical coupling.

In some cases, the optical coupling between first guided optical mode150 and optical waveguide 130 may be reduced by reducing the spacingbetween optical waveguides 120 and 130 by, for example, moving opticalwaveguide 130 closer to optical waveguide 120 as shown schematically inFIG. 5B. In some cases, core 112 of microresonator 110 has a center 117and a radius r₀. In such cases, center 117 is spaced a distance P₁ froma closer edge 508A of core 132 and a distance P₂ from a farther edge508B of core 132. Core 132 of optical waveguide 130 has a width W₁ thatis equal to P₂−P₁. In some cases, P₂ is less than or equal to r₀. Insome cases, the spacing between optical waveguides 120 and 130 is lessthan 2r₀.

In some cases, optical device 100 may have more than two opticalwaveguides. For example, FIG. 5C shows a schematic top-view of anoptical device 590 having three optical waveguides 120, 130, and 570. Inparticular, core 122 of first optical waveguide 120, core 132 of secondoptical waveguide 130, and core 572 of a third optical waveguide 570each extends from core 112 of microresonator 110.

In some cases, optical device 100 can be capable of detecting a changein the index of refraction of top cladding layer 114. For example, topcladding layer 114 may initially be air resulting in launching of guidedmode 150 when light from light source 140 enters first waveguide 120. Achange in the index of refraction of top cladding layer 114 can occurwhen, for example, the air cladding is replaced by or mixed with, forexample, a vapor, such as an organic vapor, a gas, a liquid, abiological or chemical material, or any other material that can resultin a change in the index of refraction of cladding 114. In some cases,the change in the index of refraction of cladding 114 can induce afrequency shift in guided optical mode 150. The frequency shift may bedetected by detector 160.

Microresonator 110 of FIG. 1 is shown to be, for example, a diskmicroresonator. In general, microresonator 110 can be any typeresonator, such as any shape microcavity, capable of supporting at leastone guided optical mode and capable of coupling to one or more opticalwaveguides. In some cases, microresonator 110 has circular symmetry,meaning that the perimeter of a cross-section of core 112 ofmicroresonator 110 can be expressed as a function of distance from acentral point only. In some cases, such as in a disk-shapedmicroresonator, the center point can be the center of the microresonatorsuch as center 117 of microresonator 110. Exemplary microresonatorshapes having circular symmetry include a sphere, a disk, and acylinder.

In some cases, microresonator 110 can have spherical symmetry such asphere-shaped microresonator. In some cases, microresonator 110 can be aclosed loop microresonator. For example, FIG. 6 shows a schematictop-view of an optical device 500 that includes a ring microresonator510 that can, in some cases, be a multimode microresonator. Forsimplicity and without loss of generality some parts of microresonator510 are not explicitly shown or identified in FIG. 6. Core 122 of firstoptical waveguide 120 extends from core 512 of microresonator 510.Similarly, core 132 of second optical waveguide 130 extends from core512 of microresonator 510. In some cases, microresonator 510 is amultimode microresonator in the radial direction.

As another example, FIG. 7 shows a schematic top-view of an opticaldevice 600 that includes a racetrack microresonator 610 that can, insome cases, be a multimode microresonator. For simplicity and withoutloss of generality some parts of microresonator 610 are not explicitlyshown or identified in FIG. 7. Core 612 of microresonator 610 has linearportions 630 and 632 and curved portions 640 and 642. Core 122 of firstoptical waveguide 120 extends from core 612 of microresonator 610.Similarly, core 132 of second optical waveguide 130 extends from core612 of microresonator 610.

FIG. 8 shows a schematic top-view of an optical device 700 that includesa microresonator 710 capable of supporting at least first and secondguided optical modes 150 and 152, respectively, where second guidedoptical mode 152 is different than first guided optical mode 150.Optical device 700 further includes an optical waveguide 720.Microresonator 710 has a core 712 and optical waveguide 720 has a core722. For simplicity and without loss of generality some parts of themicroresonator and the optical waveguide, such as the cladding(s), arenot explicitly shown or identified in FIG. 8.

Waveguide core 722 has an input face 722A that is in opticalcommunication with light source 140. The other end of core 722terminates at port 715A of core 712. Optical waveguide 720 and port 715Aare so arranged relative to each other and core 712 that lightpropagating along the positive y-direction in optical waveguide 720,such as light 701, is capable of coupling primarily to first guidedoptical mode 150 but not second guided optical mode 152 ofmicroresonator 710. Optical waveguide 720 and port 715A are furthermoreso arranged that light propagating along the negative y-direction inoptical waveguide 720, such as light 702, is capable of couplingprimarily to second guided mode 152 but not first guided optical mode150 of microresonator 710.

In some cases, microresonator 710 has circular symmetry. In some cases,a guided mode of microresonator 710, such as guided optical mode 150, iscapable of propagating within microresonator 710 while maintaining asame electric field profile.

Light source 140 is capable of emitting light 142. At least a portion oflight 142 enters optical waveguide 720 through input face 722A of thewaveguide and propagates along the positive y-axis as light 701. In somecases, light 701 can be a guided mode of optical waveguide 720. At port715A, light 701 optically couples primarily to and launches first guidedoptical mode 150 of the microresonator. In some cases, light 701 mayweakly couple to and launch second guided optical mode 152, but any suchcoupling will be weak and secondary to the optical coupling betweenlight 701 and first guided optical mode 150. For example, if light 701launches both guided modes 150 and 152, guided mode 150 will besubstantially more intense than guided mode 152.

When scattering center 170 is brought into optical proximity withmicroresonator 710, the scattering center induces an optical scatteringbetween first guided optical mode 150 and second guided optical modes152, resulting in a transfer of energy from guided mode 150 to guidedmode 152. If guided mode 152 is currently excited in microresonator 710,then the scattering center results in a stronger and more intense guidedoptical mode 152. If guided mode 152 is not already present inmicroresonator 710, then scattering center 170 induces a launching ofguided mode 152 by causing optical scattering from first guided mode 150into second guided mode 152.

Guided optical mode 152 optically couples to optical waveguide 720 bycore coupling and propagates inside the waveguide as light 702 towardinput face 722A. Optical element 730 redirects at least a portion oflight 702 as light 703 towards detector 160. Detector 160 detects thetransfer of energy between guided modes 150 and 152 and by doing so, iscapable of detecting the presence of scattering center 170.

Optical element 730 redirects by, for example, reflection at least aportion of light 702 along the x-axis while transmitting at least aportion of input light 142. Optical element 730 can be a beam splitter.As another example, optical element 730 can be an optical circulator.

In the exemplary optical devices shown in FIGS. 1-8, the opticalwaveguides extend linearly. In general, an optical waveguide coupled toa microresonator can have any shape that may be desirable in anapplication. For example, in optical device 800 shown schematically inFIG. 9, optical waveguides 120 and 130 have curved portions, such ascurved portions 801 and 802. Core 132 of waveguide 130 intersects core112 of microresonator 110 at an attachment location 815. The anglebetween cores 132 and 112 is β₃ defined as the angle between line 810tangent to core 132 at location 815 and line 820 tangent to core 112 atthe same location.

In some cases, the curvature of a curved portion of a waveguide issufficiently small that the curvature results in no or little radiationloss. In some cases, an optical waveguide coupled to a microresonatorcan be a nonlinear waveguide, a piecewise linear waveguide, or awaveguide that has linear and nonlinear portions.

In the exemplary embodiment shown in FIGS. 1-2, cores 122, 112, and 132are substantially in the same plane. In such a case, the core couplingbetween an optical waveguide and a microresonator can be considered tobe a lateral core coupling. For example, optical waveguide 120 laterallycore couples to microresonator 110 in core coupling region 122B.Similarly, optical waveguide 130 laterally core couples tomicroresonator 110 in core coupling region 132A.

In some cases, a microresonator core and an optical waveguide core maybe in substantially different planes. In such cases, the core couplingbetween the optical waveguide and the microresonator may be consideredto be a vertical core coupling. For example, FIGS. 12A and 12B arerespective schematic top- and side-views of an optical device 1200 inwhich microresonator core 112 is positioned in a plane PL1 and opticalwaveguide cores 122 and 132 are positioned in a plane PL2 different thanplane PL1. Optical waveguide 120 vertically core couples tomicroresonator 110 in a core coupling region 1201, where region 1201 isthe region of overlap between cores 112 and 122. Similarly, opticalwaveguide 130 vertically core couples to microresonator 110 in a corecoupling region 1202, where region 1202 is the region of overlap betweencores 112 and 132.

In some applications, microresonator 110 and optical waveguides 120 and130 in optical device 1200 can form a unitary construction and can befabricated using known fabrication methods such as a molding process.

In the exemplary embodiment shown in FIGS. 12A and 12B, opticalwaveguides 120 and 130 end at termination points 1201A and 1202A,respectively. In general, an optical waveguide may terminate at anylocation as long as there is an area of overlap between themicroresonator core and the optical waveguide core to allow for verticalcore coupling. For example, FIG. 12C shows a schematic top-view of anoptical device 1250 in which optical waveguide cores 122 and 132 extendacross microresonator core 112.

In the exemplary embodiment shown in FIGS. 12A and 12B, opticalwaveguide cores 122 and 132 are located below microresonator core 112.In general, an optical waveguide that vertically core couples to amicroresonator may be positioned above or below the microresonator. Forexample, FIGS. 13A and 13B show respective schematic top- and side-viewsof an optical device 1300 in which microresonator core 112 is positionedin a plane PL1 and optical waveguide cores 122 and 132 are positioned ina plane PL2 positioned above plane PL1. Optical waveguides 120 and 130vertically core couple to microresonator 110 in core coupling regions1201 and 1202, respectively.

In general, an optical waveguide core is so oriented relative to amicroresonator core to allow coupling of light between the opticalwaveguide and the microresonator by a core coupling. For example, FIG.14 shows a schematic three-dimensional view of an optical device 1400 inwhich optical waveguide 120 core couples to microresonator 110 in corecoupling region 1401 and optical waveguide 130 core couples tomicroresonator 110 in core coupling region 1402.

Some of the advantages of the disclosed embodiments are furtherillustrated by the following example. The particular materials, amountsand dimensions recited in this example, as well as other conditions anddetails, should not be construed to unduly limit the present invention.An optical device similar to optical device 100 of FIGS. 1 and 2 wasnumerically analyzed using an effective two dimensional FiniteDifference Time Domain (FDTD) approach. For the simulation,microresonator core 112 was silicon in the shape of a disk having a corediameter D equal to 3.6 microns, a core thickness h₁ of 0.2 microns, anda core index of refraction of 3.5. Cores 122 and 132 were each siliconwith a thickness of 0.2 microns and an index of refraction of 3.5. Uppercladding 114 was water with an index of refraction equal to 1.33. Lowercladding 165 was silicon dioxide with a thickness of 3 microns and anindex of refraction of 1.46. Substrate 161 was dilicon with an index ofrefraction equal to 3.5.

Light source 140 was a pulsed light source emitting light 142 in theform of discrete 1 femtosecond long Gaussian pulses centered atwavelength 2 microns with a full width at half maximum (FWHM) of 1.5microns. The broadband input pulses resulted in a wide spectrum responsein the range from about 1 micron to about 3 microns detected by detector160.

FIG. 10A shows the calculated signal strength (in arbitrary unitsrelative to the intensity of input light) at detector 160 as a functionof wavelength (in microns). Curve 910 shows the signal strength in theabsence of scattering center 170. Curve 920 shows the signal strength inthe presence of scattering center 170. In generating curve 920, thescattering center was a solid spherical gold nanoparticle in physicalcontact with microresonator 110. Scattering center 170 had real andimaginary indices of refraction of 0.54 and 9.58, respectively. Thediameter of the scattering center was 80 nanometers. Curves 910 and 920show that the output spectrum at detector 160 changes substantially whenscattering center 170 is brought into contact with the microresonator.

FIG. 10A shows that the microresonator has high Q-factors at severalwavelengths. For example, the numerical analysis showed that theQ-factor of the microresonator was 1424 at 1.43 microns, 1240 at 1.49microns, and 781 at 1.56 microns. Q-factor can be defined as λ₀/Δλ₀where λ₀ is the center (resonant) wavelength and Δλ₀ is the full widthat half maximum (FWHM).

FIG. 10B is an expanded view of curves 910 and 920 near 1.56 micronsshowing that scattering center 170 resulted in a relatively large shiftof about 0.7 nanometers from peak 911 to peak 912. An advantage of thedisclosed embodiments is that the presence of a scattering center inoptical proximity to a microresonator can result in a relatively largeshift in a peak of the output spectrum at detector 160. In some cases,the shift can be greater than 0.1 nanometers, or greater than onenanometer, or greater than 2 nanometers, or greater than 5 nanometers.

In some applications, light source 140 can be a broadband light sourceemitting, for example, white light. Similarly, detector 160 can be abroadband detector. In such cases, detector 160 can signal the presenceof a scattering center if the overall detected light intensity is abovea pre-determined intensity threshold. For example, referring to FIG.10A, a detected intensity level larger than an intensity threshold level930 set at about 0.16 can indicate the presence of scattering center170. In some cases, the threshold level can be set a lower level, suchas at a threshold level 930A, to include a small fraction of signalstrength 910 while rejecting a substantial portion of signal strength910 as background noise. An advantage of broadband light sources anddetectors is reduced overall device cost.

FIG. 11 shows a schematic three-dimensional view of an integratedoptical device 1100. Light source 140 and detector 160 are integratedonto substrate 161 of optical device 1100. Light source 140 is separatedfrom waveguide 120 by a gap 1101 and includes electric leads 1140 and1141 integrated onto substrate 161. Electric leads 1140 and 1141 extendto an edge 1121 of optical device 1100 for connection to, for example,an external power source and/or a controller not shown in FIG. 11.Detector 160 is separated from waveguide 130 by a gap 1102 and includeselectric leads 1130 and 1131 integrated onto substrate 161. Electricleads 1130 and 1131 extend to an edge 1122 of optical device 1100 forconnection to, for example, an external power source and/or otherelectronics not shown in FIG. 11.

In some applications, a light detector, such as a camera 1160, may beemployed to monitor the optical intensity level in a particular area ofmicroresonator 110. For example, camera 1160 can image and monitor thelight intensity magnitude and/or profile in an area 1110 near, forexample, the center of microresonator 110. In some cases, area 1110 canbe capable of extracting light from the microresonator. For example,area 1110 can be roughened or structured to scatter light. As anotherexample, area 1110 can be coated with a high index material to allowlight extraction in that area. In such a case, camera 1160 may be placedin direct contact with the high index material.

In the absence of a light scattering center, the light intensity levelin area 1110 can be quite low. For example, the guided modes propagatingwithin the microresonator can be

WGMs substantially confined to the sides of core 112 of microresonator110. When a scattering center is bought into optical contact with a sideof the microresonator, the scattering center can scatter light that ispropagating within the microresonator in different directions includingtowards area 1110. Area 1110 can receive and out couple the scatteredlight towards camera 1160. Camera 1160 can detect the presence of thescattering center by detecting the out coupled light.

Microresonator 110 and optical waveguides 120 and 130 can be made usingknown fabrication techniques. Exemplary fabrication techniques includephotolithography, printing, casting, extrusion, and embossing. Differentlayers in optical device 100 can be formed using known methods such assputtering, vapor deposition, flame hydrolysis, casting, or any otherdeposition method that may be suitable in an application.

Substrate 161 can be rigid or flexible. Substrate 161 may be opticallyopaque or transmissive. The substrate may be polymeric, a metal, asemiconductor, or any type of glass. For example, substrate 161 can besilicon. As another example, substrate 161 may be float glass or it maybe made of organic materials such as polycarbonate, acrylic,polyethylene terephthalate (PET), polyvinyl chloride (PVC), polysulfone,and the like.

Examples of scattering centers that can be appropriate for use with thedisclosed embodiments include silicon nanoparticles and metalnanoparticles, including gold and aluminum nanoparticles. In some cases,a scattering center may be a semiconductor such as Si, GaAs, InP, CdSe,or CdS. For example, a scattering center can be a silicon particlehaving a diameter of 80 nanometers and an index of refraction (the realpart) of 3.5 for a wavelength of interest. Another example of ascattering center is a gold particle having a diameter of 80 nanometersand an index of refraction of 0.54+9.58i for wavelengths near 1550 nm.Another example of a scattering center is an aluminum particle having adiameter of 80 nanometers and an index of refraction of 1.44+16.0i forwavelengths near 1550 nm.

In some cases, the scattering center can be a dielectric particle. Insome cases, the scattering center can be a fluorescent particle. In someother cases, the scattering center can be a non-fluorescent particle.

In some cases, the size of scattering center 170 is no greater than 1000nanometers, or no greater than 500 nanometers, or no greater than 100nanometers.

As used herein, terms such as “vertical”, “horizontal”, “above”,“below”, “left”, “right”, “upper” and “lower”, and other similar terms,refer to relative positions as shown in the figures. In general, aphysical embodiment can have a different orientation, and in that case,the terms are intended to refer to relative positions modified to theactual orientation of the device. For example, even if the constructionin FIG. 2 is inverted as compared to the orientation in the figure,lower cladding 165 is still considered to be “below” upper cladding 114.

While specific examples of the invention are described in detail aboveto facilitate explanation of various aspects of the invention, it shouldbe understood that the intention is not to limit the invention to thespecifics of the examples. Rather, the intention is to cover allmodifications, embodiments, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

1. An optical device comprising: a microresonator having a circularsymmetry, the microresonator having a core; and an optical waveguidehaving a core, the waveguide core terminating at the core of themicroresonator.
 2. The optical device of claim 1, wherein themicroresonator is a disc.
 3. A sensor comprising: the optical device ofclaim 1; and a light source and a detector each capable of opticallycommunicating with the optical waveguide.
 4. The optical device of claim3, wherein the light source is a broadband light source.
 5. The opticaldevice of claim 3, wherein the detector is a broadband detector.
 6. Theoptical device of claim 1, wherein the microresonator is capable ofsupporting first and second guided optical modes, the second guidedoptical mode being different than the first guided optical mode, andwherein the optical waveguide is positioned so that light propagatingalong a first direction in the optical waveguide is capable of couplingprimarily to the first but not the second guided optical mode of themicroresonator, and light propagating along a second direction oppositeto the first direction in the optical waveguide is capable of couplingprimarily to the second but not the first guided optical mode of themicroresonator.
 7. The optical device of claim 7, wherein at least oneof the first and second guided optical modes is capable of propagatingwithin the microresonator while maintaining a same electric fieldprofile.